The present invention relates generally to hardcopy devices and methods, particularly but not exclusively to inkjet printers and to methods of operating such devices.
As is well known in the art, conventional inkjet printers generally employ one or more inkjet cartridges, often called xe2x80x9cpensxe2x80x9d, which eject drops of ink onto a page or sheet of print media. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to Hewlett-Packard Company. Historically, the pens have usually been mounted on a carriage, which is arranged to scan across a scan axis relative to a sheet of print media as the pens print a series of individual drops of ink on the print media. The series of drops collectively form a band or xe2x80x9cswathxe2x80x9d of an image, such as a picture, chart or text. Between scans, the print medium is advanced relative to the scan axis. In this manner, an image may be incrementally printed.
Over recent years the importance placed on the throughput of inkjet printers has risen dramatically. Throughput is generally measured as the number of pages of a given size, or the area of print media that a printer may ink in a given time.
One approach to increasing the throughput of such printers is to use one or more static arrays of print nozzles which span the width of pages to be printed on. Pages of print media may then be loaded onto a belt or a drum and transported under successive page wide arrays of print nozzles, or print bars. Although such page wide array systems offer the possibility of increased throughput, they suffer from certain disadvantages.
Both belt and drum based page wide array systems may be bulky, making them unsuitable for certain operating environments. Because of the possibility of air locks obstructing inkjet nozzles, inkjet nozzles generally only function correctly in a certain range of orientations; i.e. when they are arranged to eject ink broadly downwards. In practice, this means that the print bars are located above a belt or drum in order to ensure that they are correctly orientated. Usually, at least one print bar is required for each ink colour which is to be printed. Conventionally, this includes at least cyan, magenta, yellow and black. In practice though further print bars may be required for redundancy reasons, or to print further coloured inks, or a fixer substance. These factors, therefore, necessitate a large belt or drum that provides sufficient space for all of the print bars to be suitably positioned relative to the belt or drum.
At the same time, page wide array systems also require a high degree of precision in their feed paths in order to ensure satisfactory print quality results. This can be difficult or costly to achieve in the case of large drum and belt based page wide array systems.
It would therefore be desirable to provide a printing device and method, which addresses the problems found in the prior art.
According to one aspect of the present invention there is provided a printing device comprising first and second platens each arranged to support a media sheet, and first and second print stations having first and second print positions respectively, said device being arranged to index said platens in a predetermined order from said first to said second print position, said device being further arranged to advance a platen located in a print position relative to said corresponding print station such that said corresponding print station may print on different areas of a sheet supported on said platen, said indexing motion and said advance motion being independently controllable.
Advantageously, this aspect of the invention decouples media advance movement and media indexing movement. That is to say that the movement that feeds print media relative to a given print bar, for example, allowing that print bar to incrementally print across the printable surface of the sheet, may be controlled separately from that which feeds print media from one print bar to the next. Thus, for example, the media advance movement may be zero when the media is being indexed and vice versa.
Various advantages follow from the configurations of printer devices according to this aspect of the invention. Firstly, smaller platens may be used. In one preferred embodiment, drum platens are used. By separating the required print bars in a page wide array system and indexing, or moving the drums from one print station to another, fewer print bars need print on a given sheet at any given time. Thus, the space required by print bars at any given print station is reduced allowing the use of smaller diameter drums.
Preferably, the drums are as small as possible such that the largest sheet for which the printer is designed to function may just fit on to an individual drum. By using small drums various further advantages are realised. Unlike a large drum, it is relatively easy and inexpensive to fabricate small drums of high tolerance. These may be manufactured using conventional manufacturing techniques, such as injection moulding. Small drums of this sort may have more inherent rigidity than corresponding larger drums. Furthermore, smaller drums may be more easily driven due to lower total inertia. At the same time, small drums offer various advantages over belts for use in page wide array systems, for example. They offer significantly better control over media handling. Higher media hold down forces, such as vacuum forces, may generally be obtained to hold the print media to the surface of a drum than of a belt. Additionally, a drum does not encourage print media to oscillate in an axial direction as it is fed in the media advance direction in the manner that belts have a tendency to do. Furthermore, a drum generally allows the distance between the writing head of a print station, for example an inkjet head, and the print medium to be more accurately maintained. This in turn often allows faster printing speeds to be achieved.
In a preferred embodiment of the present invention, a given sheet is supported on a single drum throughout the process in which it is printed on. This enables the sheet to be more securely held, which reduces the risk of movement between the sheet and the platen or drum of the printer. Thus, the likelihood of print defects, such as registration defects, is also reduced. It will be understood that a printing process may comprise the printing of several colour separations and the application of fixer, which may be applied by a print bars, for example, located at a number of print stations.
Preferably, the advance motion of the platen relative to said corresponding print station is arranged such that the corresponding print station may print over substantially the whole printable area of the sheet supported on said platen in one continuous operation. This may allow for relatively rapid printing with correspondingly high levels of throughput.
A further advantage which follows from the configurations of printer devices according to this aspect of the invention is that the number of print passes which may be made with each print bar over a give sheet of print media may to be controlled in a flexible way. In the case of a preferred embodiment of the invention two or more drums platens are used. In such an embodiment, the media advance movement may be provided by causing the drums to rotate about their own longitudinal axes. The media indexing movement may be provided by causing an assembly of drum platens to rotate about a central hub, to successively bring the drums to printing positions adjacent to different print bars. In this manner, by varying the number of media advance revolutions made by the drums in between successive indexing movements, the number of print passes may be varied. Alternatively, if the platens follow a closed loop indexing path, selected sheets may pass around the indexing path one or more times. Each extra time the sheets pass around the path, the number of passes is increased.
Furthermore, the same methods of altering the pass number may alternatively or instead be used to provide redundancy for the print bars. This approach to redundancy may be beneficial in terms of hardware cost and space constraints when compared to the conventional use of duplicate print bars.
The present invention extends to the corresponding method. In another aspect, the present invention also extends to a computer program, arranged to implement the method of the present invention.